The present invention is directed to a process for controlling the triboelectric properties of toners. More specifically, the present invention is directed to a process for controlling the triboelectric properties of toner particles by preparing a polymeric core material containing the pigment colorant and encapsulating the core material within a polymeric shell.
Toners suitable for use in electrophotographic copiers and printers may have a wide variety of colors, such as black, red, green, blue, brown, yellow, purple, silver and gold. When it is desired to highlight certain features of a document, one or more colored toners are typically used in conjunction with a black toner to provide an image in two or more colors. Full color images can also be produced by developing images with cyan, magenta, yellow, and black toners. Generally, it is advantageous for such toners to exhibit low melting temperatures to enable low energy fusing of the developed images to substrates at lower temperatures. It is also often advantageous for such toners to possess mean particle diameters of less than about 10 microns to enable images of high resolution, low image noise and high color fidelity. Further, it is generally desirable for these small diameter toners to have very narrow size distributions, preferably in the range of a geometric size dispersity (GSD) of 1.2 to 1.3, to avoid difficulties in electrophotographic development and transfer associated with oversize toner particles and extremely fine toner particles.
It is also an advantage to be able to control selectively the triboelectric charging level of different color toners, permitting them to attain the same equilibrium triboelectric charging level against selected carriers in two component development systems. It is also desirable to be able to control the rate of charging of toner, freshly added to a toner depleted development package, such that it rapidly achieves the same sign and level of triboelectric charge possessed by the equilibrated toner.
When manufacturing color toners by conventional techniques, which include melt-blending the colored pigments into desired resins and employing Banbury mixers or screw extruders, several problems are encountered in the preparation of colored toners with controlled triboelectric properties. First, the quality of the pigment dispersion greatly influences the triboelectric position of the toner. While pigment dispersion agents can improve the pigment dispersion and thereby improve the transparency of the toner, these dispersants can adversely affect the triboelectric properties of the toner and the triboelectric stability at different relative humidities and can also interfere with the melt rheological properties and fusing properties of the toner particles.
In addition, when attempting to prepare very small diameter toners with narrow size distributions by micronization and classification of melt-blended composites, there is generally a significant loss of yield of toner due to creation of fines, which generally are toner particles with an average particle diameter of less than 5 microns. This problem is aggravated when attempting to prepare low melting/low fusing temperature compositions, since the jetting yield is reduced for soft, rubbery materials. Although this problem can be overcome to some extent by cryogenic jetting, using cold air, it is only at the expense of slower jetting rates and increased costs resulting from the need to employ cooled air.
Further, it is frequently observed that different colored pigments, as a result of their complex and unique molecular structures, have a profound effect on the triboelectric properties of toners prepared by melt blending the pigments with preferred resins, followed by micronization and classification. Thus, certain pigments can dominate the triboelectric properties to the extent that two different pigments, such as Lithol Scarlet D3700 (BASF) and PV Fast Blue B2G01 (American Hoechst), when melt-blended into an identical resin at identical pigment loadings and formulated into toners of equivalent particle diameter and particle size distribution, will exhibit dramatically different triboelectric properties. For example, a toner composition comprising from about 5 percent to about 8 percent by weight of Lithol Scarlet pigment in from about 92 percent to about 95 percent by weight of a styrene-butadiene copolymer resin comprising about 87 percent by weight of styrene and about 13 percent by weight of butadiene is typically characterized by a triboelectric charge of from about -30 to -40 microcoulombs per gram. In contrast, a toner composition comprising the same amount of PV Fast Blue pigment in the same styrene-butadiene resin is typically characterized by a triboelectric charge of about +50 microcoulombs per gram. This pigment domination effect may be overcome to some extent through the use of added charge control agents, either negative or positive, but the effect is generally not rectified completely and is very difficult to control. It is generally advantageous for optimum image development to be able to achieve identical levels of triboelectric charging, employing the same resin system and the same carrier, for toners prepared from different colored pigments. This result would allow for simpler, more reliable, and reproducible developer subsystems and fuser subsystems and would result in improved copy quality.
The process of the present invention provides the aforementioned advantages by providing a process for controlling the triboelectric properties of colored toner particles which comprises preparing a first core material comprising first pigment particles, core monomers, a free radical initiator, and optional polymer components; preparing a second core material which comprises second pigment particles, core monomers, a free radical initiator, and optional polymer components, said second pigment particles being of a different color from that of the first pigment particles; dispersing the first and second core materials into an aqueous phase; encapsulating separately the first core material and the second core material within polymeric shells by means of interfacial polymerization reactions between at least two shell monomers, of which at least one is soluble in aqueous media and at least one of which is soluble in organic media, wherein the polymeric shell encapsulating the first core material is of substantially the same composition as the polymeric shell encapsulating the second core material; and subsequently polymerizing the first and second core monomers via free radical polymerization, thereby producing two encapsulated heat fusible toner compositions of different colors with similar triboelectric charging characteristics.
Encapsulating toners are known. For example, U.S. Pat. No. 4,565,764 discloses a pressure fixable microcapsule toner having a colored core material coated successively with a first resin wall and a second resin wall. The first resin wall has affinity to both the core material and the second resin wall. This patent teaches that the first resin wall may be of a material that becomes charged to a polarity opposite to that of the second resin wall and the core material.
Additionally, U.S. Pat. No. 4,520,091 discloses a pressure fixable encapsulated electrostatographic toner material. The core comprises a colorant, a polymer, a solvent capable of dissolving the polymer or causing the polymer to swell, and an organic liquid incapable of dissolving the polymer or causing the polymer to swell, while the shell may consist of a polyamide resin. Preparation of the toner material is completed by interfacial polymerization.
Another patent, U.S. Pat. No. 4,708,924, discloses a pressure fixable microcapsule type toner composed of a core material and an outer wall covering over the core material. The core material contains at least a combination of a substance having a glass transition point within the range of -90.degree. C. to 5.degree. C. with a substance having a softening point within the range of 25.degree. C. to 180.degree. C. This toner composition may comprise substances such as polystyrene and poly(n-butyl)methacrylate and their copolymers.
Further, U.S. Pat. No. 4,254,201 discloses a pressure sensitive adhesive toner consisting essentially of porous aggregates. Each aggregate consists essentially of a cluster of a multiplicity of individual granules of pressure sensitive adhesive substance, each granule being encapsulated by a coating film of a film-forming material. Particles of an inorganic or organic pigment and/or a magnetic substance are contained within the aggregate in the interstices between the granules and deposited on the surfaces of the encapsulated granules. The adhesive substance is selected from a copolymer of at least one monomer and as many as three other monomers.
In addition, U.S. Pat. No. 4,702,988 discloses a process for producing toner. A monomer composition and a colorant are dispersed in a liquid dispersion medium in the presence of a solid fine powdery dispersion stabilizer. The liquid is pressurized and then ejected into a low pressure section to form particles of monomer composition. These particles are then subjected to suspension polymerization to produce toner particles.
Further, U.S. Pat. No. 4,766,051 discloses colored cold pressure fixable toner compositions with hard shells obtained by hydrolysis and interfacial polymerization. The core consists of the organic soluble shell component or components, a core polymer, a low boiling point solvent into which the core polymer is soluble and a dispersed newsprint ink concentrate. The newsprint inks are inexpensive rubber based printing inks consisting of cyan, magenta, yellow, red, and mixtures thereof excluding carbon black and magnetite.
U.S. Pat. No. 4,727,011 discloses a process for preparing encapsulated toner compositions which comprises mixing, in the absence of a solvent, a core monomer, an initiator, pigment particles, a first shell monomer, stabilizer, and water; thereafter adding a second shell monomer, thereby enabling an interfacial polymerization reaction between the first and second shell monomers; and subsequently effecting a free radical polymerization of the core monomer. The disclosure of this patent is totally incorporated herein by reference.
Further, U.S. Pat. No. 4,766,051 discloses an electrophotographic developer composition comprising a cold pressure fixable colored toner composition which comprises a core containing a polymer in which is dispersed pigment particles selected from the group consisting of cyan, magenta, red, yellow pigments, and mixtures thereof, other than carbon blacks and magnetites; and encapsulated within a polymeric shell formulated by an interfacial polymerization. Also, U.S. Pat. No. 4,725,522 discloses a process for preparing cold pressure fixable toner compositions which comprises admixing a core component comprising pigment particles, a water insoluble organic solvent and elastomeric materials with a shell monomer dissolved therein, dispersing the resulting mixture in a water phase. In addition, U.S. Pat. No. 4,766,051, the disclosure of which is totally incorporated herein by reference, discloses a cold pressure fixable colored toner composition comprising a core containing a polymer in which is dispersed colored pigment particles and an encapsulating polymeric shell formulated by an interfacial polymerization.
In addition, U.S. Pat. No. 4,628,019 discloses a single component, substantially nonmagnetic toner which comprises a colorant and a binder, said binder preferably comprising a copolymer of an aminoacrylic monomer and a vinyl monomer having a (M.sub.w /M.sub.n) (weight-average molecular weight (M.sub.w) to number-average molecular weight (M.sub.n) ratio) of less than 5.0. According to the teachings of this patent, the charging characteristics of the toner are improved by decreasing the M.sub.w /M.sub.n ratio of the binder in that the quantity of static charge possessed by the toner is stabilized. As stated at column 5, lines 50 to 56, the charging characteristic of the resin which is the main component of the toner has a fundamental effect on stabilization of the charging characteristic of the toner, and the charging characteristic of the toner is improved by decreasing the M.sub.w /M.sub.n ratio of the binder resin. The examples in this patent indicate that lowering the M.sub.w /M.sub.n decreases the triboelectric charge fluctuations observed in the toners when the amount of iron or ferrite powder in the toner is varied. As stated at column 5, lines 44 to 47, the toner composition disclosed in the patent may also be adopted in the wall material, the core material, or both of a microcapsule toner.
Although known processes do not provide methods of passivating the toner pigments so that triboelectric charge is independent of the pigment contained therein, processes for controlling triboelectric characteristics of toners are known. For example, U.S. Pat. No. 4,613,559 discloses a process for obtaining colored toner compositions by dispersion polymerization, which comprises providing a monomer solution containing stabilizer, polymerizing the resulting mixture, adding to the mixture a dye solution comprising an organic solvent and oil soluble dyes, causing the dye solution to diffuse into the polmer particles, and separating the resulting toner particles, wherein the stabilizer is permanently attached to the toner polymer particles. The steric stabilizers that are permanently attached to the toner particle surfaces function as charge enhancing additives, and changing the stabilizer selected enables a triboelectric charging range of the toners disclosed of from -50 to +50 microcoulombs per gram.
In addition, U.S. Pat. No. 4,134,760, the disclosure of which is totally incorporated herein by reference, discloses developer compositions wherein the triboelectric charging potential of functional polymers employed in the toner materials are controlled through chemical acylation of hydroxyl and amino functions. The controlled variation of the triboelectric behavior of functional polymers by acylation provides a means of attaining optimum triboelectric responses in development systems. By varying the degree of chemical modification of polymeric materials for use as toner particles, either stoichiometrically or kinetically, the triboelectric properties of the developer material are controlled in a continuous manner.
Further, U.S. Pat. No. 4,070,296 discloses developer compositions wherein the triboelectric charging properties of functional polymers employed in the toner materials are controlled by systematic chemical modification. The functionalized polymers are covalently bonded with functional dyes to provide colored toner materials possessing controlled triboelectric properties and stable colorants. Additionally, U.S. Pat. No. 4,070,186 discloses developer compositions wherein the triboelectric charging potential of functional polymers employed in the toner materials are controlled through chemical alteration of active hydrogen containing materials by silylation. The controlled variation of the triboelectric behavior of functional polymers by silylation provides a means of attaining optimum triboelectric responses in development systems.
Copending application U.S. Ser. No. 043,265, filed 4/27/87, discloses an encapsulated composition suitable for use as an electrophotographic toner, which comprises a core encapsulated within a thermotropic liquid crystalline polymeric shell. On page 8 of this application, the specification states that the disclosed developer compositions can be charged to preselected values irrespective of the pigment selected for the core. This teaching, however, refers only to toner compositions comprising core components and liquid crystalline polymeric shells. In addition, copending application U.S. Ser. No. 128,851, filed 12/4/87, discloses an encapsulated toner composition with a melting temperature of from about 65.degree. C. to about 140.degree. C. which comprises a core containing a polymer selected from the group consisting of polyethylene succinate, polyhalogenated olefins, poly(.alpha.-alkylstyrenes), rosin modified maleic resins, aliphatic hydrocarbon resins, poly(.epsilon.-caprolactones), and mixtures thereof; and pigment particles, where the core is encapsulated in a shell prepared by interfacial polymerization reactions. The disclosure of this copending application is totally incorporated herein by reference containing a stabilizing material, hydrolyzing by heating the resulting mixture, subsequently effecting an interfacial polymerization of the mixture, and thereafter optionally washing the resulting toner composition.
Further, U.S. Pat. No. 4,855,209, filed 6/24/88, discloses an improved process for preparing encapsulated toner compositions which comprises mixing core monomers, an initiator, pigment particles, and oil soluble shell monomers, homgenizing the mixture into an aqueous surfactant solution to result in an oil-in-water suspension enabling an interfacial polymerization reaction between the oil soluble and the water soluble shell monomers, subsequently adding a low molecular weight polyethylene oxide surfactant protective colloid, and thereafter effecting free-radical polymerization of the core monomers by heating. The disclosure of this U.S. Pat. No. 4,851,318 is totally incorporated herein by reference.
Although these compositions and processes are suitable for their intended purposes, a need continues to exist for improved heat fusible color toners suitable for use in electrophotographic copiers and printers. A need also exists for colored toners which exhibit low melting behavior, thereby enabling lower fusing temperatures. A further need exists for dry colored toners having an average mean diameter of less than 10 microns and a narrow size distribution. There is a further need for colored toners which charge positively or negatively in two component development, as well as a need for both positively and negatively charged toners for single component development systems. In addition, there is a need for processes for preparing heat fusible colored toners wherein the triboelectric characteristics of the toners may be controlled and predetermined. Further, a need exists for toner compositions having highly stabilized pigment dispersions with a wide choice of pigments for highlight and process color. In addition, a need exists for a process that enables colored toner particles to possess predetermined triboelectric charging characteristics independent of the pigment selected as a colorant. A further need exists for a process which enables production of toner particles with a mean particle diameter of less than 10 microns and a narrow size distribution without the need for micronization or classification. There is also a need for processes for preparing toners of different colors that can reach the same equilibrium levels of triboelectric charge when charged against the same carrier. In addition, there is a need for processes for preparing toners wherein the triboelectric charge of the toner is primarily determined by the shell material and/or by any charge control agents present. Further, a need exists for processes for preparing toners wherein the toner color can be modified without affecting the triboelectric charge of the toner.